DC-DC flyback converters using synchronous rectifiers are well known. When isolation between the input and output stage is required, the output voltage can be sensed by various methods for regulation feedback. Some ways to convey the output voltage while maintaining isolation include using an optocoupler or using a third winding on the primary side of the transformer. However, those ways require additional circuitry, space, power, and cost. A more elegant way of detecting the output voltage is to sense a voltage at a terminal of the power switch when the power switch is turned off during the discharge (or flyback) cycle of the converter. Such a sensed voltage is substantially proportional to the output voltage. However, such a scheme requires a minimum duty cycle in order for the sensing to be accurate, since current must flow in the secondary winding in order to create the primary side sense voltage. Such a scheme also generally requires a minimum load in the form of a load resistor so as to draw a minimum current during the discharge cycle in the event the actual load is in a standby mode drawing little or no current.
If there were no minimum load resistor and the actual load went into a very light current standby mode, the minimum duty cycle may be greater than that needed to achieve a regulated output voltage, and the output voltage would exceed the desired regulated level. Thus, the minimum load current must be above a threshold current to prevent this. The minimum load reduces the efficiency of the converter.
FIG. 1 illustrates one type of flyback converter 10 using a minimum load and which detects the output voltage VOUT by detecting the voltage at the primary winding when the power switch MOSFET M1 is turned off during the discharge (or flyback) cycle. No optocoupler or third winding is used to detect VOUT.
A transformer 12 has a primary winding L1 and a secondary winding L2. The MOSFET M1 is controlled by an output regulation and control circuit 14 to connect the winding L1 between the input voltage VIN (e.g., a battery voltage) and ground during a charging cycle.
To achieve a regulated VOUT, the MOSFET M1 is turned off after a controlled time, and the synchronous rectifier MOSFET M2 is turned on. The current through winding L2 is transferred to the load and the smoothing capacitor C1 at the required voltage.
For regulation feedback, the circuit 14 detects the voltage at the drain of MOSFET M1 during the discharge cycle (MOSFET M1 is off). Sensing an output voltage by a signal at the primary side of the transformer is sometimes referred to as primary side sensing. The drain voltage is related to a winding ratio of L1 and L2, and the voltage across winding L2 is the output voltage Vout plus the voltage drop across MOSFET M2 (assuming MOSFET M2 is on). The user selects the value of a feedback resistor RFB and the value of a reference resistor RREF such that (RFB/RREF)*Vref equals the desired regulated voltage, where Vref is an internal bandgap reference voltage applied to an internal error amplifier. Such primary side sensing circuits for detecting VOUT are well known and need not be described in detail. The full data sheet for the Linear Technology LT3573 flyback converter, incorporated herein by reference and available on-line, describes the operation of the feedback circuit. This operation is also described in U.S. Pat. Nos. 7,471,522 and 7,463,497, assigned to the present assignee and incorporated herein by reference. Other known primary side voltage sensing techniques may be used.
The circuit 14 continues to control the duty cycle of MOSFET M1, at a variable frequency or a fixed frequency, to regulate VOUT based on the sensed voltage.
The circuit 14 may also directly control the synchronous rectifier MOSFET M2 to turn on when MOSFET M1 turns off, or an automatic synchronous switch control circuit 16 may control MOSFET M2 to turn on at the proper times. MOSFETs M1 and M2 are typically never on at the same time. The diode D2 represents the drain-body diode of the MOSFET M2.
The output regulation and control circuit 14 may use any type of conventional technique to regulate, including current mode, voltage mode, or other modes.
When the load is above a certain threshold current, conventional operation of the converter 10 is used to accurately regulate VOUT. However, when the actual load falls below the threshold current, the required minimum duty cycle of the converter 10 generates too much current and causes VOUT to rise above the regulated voltage. Such light load operation still requires a minimum duty cycle to sample the output voltage on the primary winding L1. In the event that the actual load is a type that has a standby mode that draws very little power, the converter 10 is provided with a minimum load current resistor R1 to help dissipate the winding L2 current so regulation can be maintained during the periodic cycling of MOSFETs M1 and M2. Alternatively, or in conjunction, a zener diode D3 is used to ensure VOUT does not rise above a threshold level. Resistor R1 and zener diode D3 are optional, since the minimum current drawn by the actual load may be sufficient to substantially maintain regulation at the lightest load current.
FIG. 2 illustrates the current through the primary winding L1, the current through the secondary winding L2, and the voltage VM1 across the MOSFET M1 for a relatively low duty cycle operation. It may be assumed that the actual load current is below the minimum current set by the minimum current load resistor R1.
At time T1, the MOSFET M1 turns on to charge the primary winding L1, causing a ramping current to flow in winding L1. MOSFET M2 is off at this time.
After a variable or fixed time, at time T2, MOSFET M1 shuts off and MOSFET M2 turns on. This may be at the minimum duty cycle. This ceases current in the primary winding L1 and causes the current through the secondary winding L2 to ramp down while charging the output capacitor C1 and providing current to the load. The voltage across the MOSFET M1 is related to the output voltage VOUT and is sampled during this time by the circuit 14. The current supplied to the capacitor C1 during this light load condition may increase VOUT beyond the avalanche voltage of the zener diode D3, clamping VOUT to that value.
At time T3, the secondary winding L2 current ramps down to zero and the MOSFET M2 turns off to cause a discontinuous mode. MOSFET M2 may be turned off by a circuit that detects a slight reversal of current through the winding L2 by detecting the voltage across MOSFET M2.
After time T3, the parasitic capacitance of MOSFET M1 and the inductance of winding L1 creates an oscillating tank circuit.
At time T4, MOSFET M1 turns on again, and the cycle repeats, which may be the minimum duty cycle.
Additional detail of various converter circuits are described in U.S. Pat. Nos. 5,481,178; 6,127,815; 6,304,066; and 6,307,356, assigned to the present assignee and incorporated herein by reference.
During a medium to high current mode of the converter 10, there may be no discontinuous operation, and the converter 10 may operate at a fixed frequency with a variable duty cycle to regulate the output voltage. Such an operation may be conventional.
During the light load condition of the load, such as a standby mode, it is important that the converter 10 draw as little current as possible to extend battery life. Such standby modes typically occur for relatively long periods. It would be desirable to not require a minimum current load circuit (e.g., resistor R1) to enable the converter 10 to regulate VOUT when the actual load is in its standby mode. By doing away with the minimum current circuit, while still achieving substantial regulation when the actual load is drawing zero or very little current, efficiency is improved and battery life is increased.